This invention relates to apparatus for mechanically coupling a transducer to a source of ultrasonic acoustic emissions, i.e., acoustic emissions at frequencies above the range of human hearing. In particular, the invention relates to apparatus for holding a transducer to sense the ultrasonic acoustic emissions of fluid flow through an orifice, such as in the fuel injector assembly of an internal combustion engine, for tachometry purposes or the like.
It is desirable to determine the timing of the injection of fuel into internal combustion engines, such as diesel engines, for purposes of analyzing engine performance or the like. It is known to determine this timing by sensing the characteristic ultrasonic acoustic emissions caused by the injection of the fuel into the cylinder, which ultrasonic acoustic emissions typically have a predominant frequency of approximately 40 Khz. For example, U.S. Pat. No. 4,444,049, discloses the use of a pliers-type clamp for clamping onto the fuel delivery nozzle, the clamp carrying a directional microphone for sensing the high frequency ultrasonic acoustic emissions caused by the fuel injection. However, the microphone has a fairly broad-band response and is susceptible to picking up ambient and background sounds and emissions, as well as the desired 40 Khz fuel injection ultrasonic acoustic emissions. Thus, the device is inherently noisy, resulting in inaccurate measurements. Additionally, the microphone is air coupled to the source of emissions, resulting in a significant loss of amplitude.
It is also known to clamp other types of transducers to the fuel injector assembly, such as acoustical magnetic transducers, as disclosed in U.S. Pat. No. 4,187,720 or piezoelectric transducers, as disclosed in U.S. Pat. No. 4,319,481. But, again, these devices are not very selective and, therefore, tend to be rather susceptible to acoustic noise. More specifically, the fuel injection apparatus and related parts of the internal combustion engine emit a number of different frequency acoustic emissions and vibrations, in addition to the 40 Khz acoustic emission which is sought to be detected. These extraneous emissions confuse the output signals from the prior art sensing devices.